1. Field of the Invention
The present invention relates to a protective coating for an article exposed to hot, aggressive gas flows and, more particularly, to a ceramic thermal barrier layer for a gas turbine engine component.
2. Description of Related Art
Gases flowing through a turbine engine reach extremely high temperatures and velocities. It is a significant engineering challenge to build components that will withstand the impingement of a high velocity gas at temperatures that can exceed 1000xc2x0 C. The demands on an engine""s turbine blades are particularly extreme, because they are exposed to high velocity, high temperature gases while being subjected to forces resulting from rotation at thousands of revolutions per minute.
Prior art turbine blades are typically a laminated structure, with a so-called superalloy substrate or base body having a heat resistant coating. These superalloys are typically cobalt- or nickel-based materials, and the protective coatings have taken a variety of forms. One known component of such coatings is an adhesion promotion layer of an MCrAlY alloy, where Cr is chromium, Al is aluminum and Y is yttrium and/or a rare-earth element, with the remainder M selected from the group consisting of iron, cobalt, nickel or mixtures thereof. That layer forms a bonding oxide for a ceramic thermal barrier layer.
U.S. Pat. No. 4,585,481 discloses protective layers for protecting a superalloy metallic substrate against high-temperature oxidation and corrosion. MCrAlY alloys are employed for the protective layers, and the patent discloses such layers with 5% to 40% chromium, 8% to 35% aluminum, 0.1% to 2% of an oxygen-active element from group IIIb of the periodic table, including the lanthanides and actinides and mixtures thereof, 0.1% to 7% silicon and 0.1% to 3% hafnium, the remainder being made up of nickel and/or cobalt. (Proportions are in percentages by weight.) The corresponding protective layers made of MCrAlY alloys are, according to this patent, applied using a plasma-spray method.
U.S. Pat. No. 4,321,310 is another example of such prior art. It describes a gas turbine component which has a base body made of the nickel-based superalloy MAR-M-200. A layer of an MCrAlY alloy, in particular an NiCoCrAlY alloy, having 18% chromium, 23% cobalt, 12.5% aluminum and 0.3% yttrium, with the remainder being made up of nickel, is applied to the base material. This alloy layer has a polished surface, to which an aluminum oxide layer is applied. A ceramic thermal insulation layer, which has a columnar structure, is applied to this aluminum oxide layer. In the columnar microstructure of the thermal barrier layer, crystallite columns stand perpendicular to the surface of the base body. Stabilized zirconium oxide is disclosed as the ceramic material.
U.S. Pat. No. 5,236,787 discloses a layer of a metal-ceramic mixture between the base body and a ceramic thermal barrier layer of an internal combustion engine valve. The metallic component of the intermediate layer increases in the direction of the base body and decreases in the direction of the thermal barrier layer, while the ceramic component is low in the vicinity of the base body and high in the vicinity of the thermal barrier layer. The thermal barrier layer is a zirconium oxide stabilized with yttrium oxide and containing cerium oxide. The object is to match the different coefficients of thermal expansion.
U.S. Pat. No. 4,764,341 describes the bonding of a thin metal layer to a ceramic to produce printed electrical circuits. Nickel, cobalt, copper and alloys of these metals are used for the metal layer. To bond the metal layer to a ceramic substrate, an intermediate oxide, such as aluminum oxide, chromium oxide, titanium oxide or zirconium oxide, is applied to the ceramic substrate. The intermediate oxide forms a ternary oxide through oxidation at a sufficiently high temperature by incorporating an element from the metallic coating.
GB 2 286 977 describes a composition for an inorganic coating for application to a low-alloy steel and being resistant to high temperatures. A main property of the coating is its resistance to corrosion, which is achieved by binding iron in the coating. Before a chemical reaction, the coating includes metal oxides which are converted into spinels at temperatures in excess of 1000xc2x0 C.
U.S. Pat. No. 4,971,839 discloses a high-temperature protection layer comprising a mixed metal oxide system which has a perovskite structure with the chemical structural formula A1-xBxMO3. In this formula, A is a metal from group IIIb of the periodic table, B is a metal from main group II (alkaline-earth metals) of the periodic table and M is a metal from one of the groups VIb, VIIb and VIIIb of the periodic table. The stoichiometric factor x is between 0 and 0.8. The coating is employed on a thermally stable steel or an alloy for use at temperatures in excess of 600xc2x0 C., in particular for a component of a gas turbine. An austenitic material based on nickel, cobalt or iron is preferably used as the component base material.
Sivakumar, R., et al., xe2x80x9cOn the Development of Plasma-Sprayed Thermal Barrier Coatings,xe2x80x9d Oxidation of Metals, Vol. 20, Nos. 3/4, pp. 67-73 (1983), disclose a variety of coatings which include a zirconate. The coatings are applied to components made of Nimonik-75 and, alternatively, an adhesion layer of the CoCrAlY type by means of plasma spraying. Results are given relating to calcium zirconates and magnesium zirconates under cyclic thermal loading.
In spite of the use of material such as partially stabilized zirconium oxide, ceramic thermal barrier layers have had a coefficient of thermal expansion which amounts to at most about 70% of the coefficient of thermal expansion of the common metallic base body made of a superalloy. Owing to the coefficient of thermal expansion of the zirconium oxide thermal barrier layer, which is lower than that of the metallic base body, thermal stresses result from exposure to a hot gas of articles with prior art protective coatings.
To counteract such stresses during thermal loading cycles, it is necessary to have an expansion-tolerant microstructure in the thermal barrier layer, for example, by setting up a corresponding porosity or a columnar structure in such layer. In the case of prior art thermal barrier layers based on partially stabilized zirconium oxide with stabilizers such as yttrium oxide, cerium oxide and lanthanum oxide, stresses resulting from a thermally induced phase transition (tetragonal to monoclinic and cubic) may occur. A concomitant change in volume dictates a maximum permissible surface temperature for zirconium oxide thermal barrier layers.
It is an object of the present invention to avoid the shortcomings of prior art structure for protecting articles in demanding environments, and particularly to provide a ceramic thermal barrier for protecting gas turbine engine components such as turbine blades.
It is another object of the present invention to provide a product having a metallic base body and a thermal barrier layer bonded thereon, in particular with a mixed metal oxide system.
In furtherance of the objects of the present invention, one aspect of the invention involves an article having a metallic substrate and a ceramic thermal barrier layer including a mixed metal oxide system comprising a compound selected from the group consisting of (i) a lanthanum aluminate and (ii) a calcium zirconate, the calcium in which is partially replaced by at least one calcium-substitute element.
In accordance with a more particular aspect of the invention, the calcium-substitute element is strontium (Sr) or barium (Ba). In addition, the lanthanum in the lanthanum aluminate can be partially replaced by at least one lanthanum-substitute element from the lanthanide group, particularly gadolinium (Gd).
In accordance with yet another aspect of the invention, a process for producing a thermal barrier layer on an article comprising a substrate for accepting the thermal barrier layer comprises the steps of providing a pre-reacted mixed metal oxide system comprising a compound selected from the group consisting of (i) a lanthanum aluminate and (ii) a calcium zirconate, the calcium in which is partially replaced by at least one calcium-substitute element, and applying the pre-reacted metal oxide system to said substrate by one of plasma spraying and an evaporation coating process.
The invention is particularly adapted for use with a component of a gas turbine engine such as a turbine blade, a guide vane or a heat shield element, in which the component substrate is a nickel-, cobalt- or chromium-based superalloy.